Tuesday, April 17, 2007

Better "Dark Frame" Subtraction in PhotoShop

David Pearson, a night photographer from Orange Park, FL, told me that he has developed a technique to automatically remove hot pixels from long exposures without turning them black.

(illustration by David Pearson)

His blog posting describes how this PhotoShop action works, and provides a link to download it for free.

I understand how it removes background pixels, but I don't understand how it would remove background noise (one of the main goals of dark frame subtraction). I haven't had a chance to discuss this with David, yet. I'm assuming that you would normally run this action to remove hot pixels, then run the standard dark frame subtraction technique to remove background noise (which must be big problem shooting at night in plus-90 degree temperatures in Florida).

3 Comments:

I have not seen any evidence of black spots from in-camera dark frame subtraction on the 20D or 5D. In-camera dark frame subtraction on the Canon 5D is not just for getting rid of hot spots -- it also gets rid of the white "pepper grain" in skies and eliminates sensor noise at the edge of the frame, all without any apparent loss of detail. The only penalty is tying up the camera for twice the time which also leads to shorter battery life.

Thanks for linking to my blog entry about dark frame subtraction. I hope it helps someone, but I'm still experimenting with it.

My technique should work with any fixed noise (i.e., the same noisy pixels in both image and dark frame) which--as I understand it--is what dark frame subtraction is all about. For random noise, the best I can do is run Noise Ninja or the like on it.

P.S. Love your blog, btw.

Joe,

I only have a lowly Digital Rebel that does not do in-camera dark frame subtraction. My guess with the higher end bodies is that they have some clever algorithm that avoids these dark spots, supresses edge sensor noise, etc. Of course, you can't create information from nothing, so the apparent preservation of detail is just that: apparent.

And btw, I still do shoot a dark frame, in case that wasn't clear. The technique requires that, just like standard dark frame techniques. Even "white 'pepper grain'" goes to black "pepper grain" with conventional dark field subtraction, btw.

Also, the beauty over in-camera dark frame subtraction is that you can create the dark frame later (given the same ambient camera temperature). Perhaps I'll shoot a looong exposure and show you. Feel free to e-mail me if I'm not being clear.